1. Field of the Invention
The invention relates to the manufacture of high density integrated circuits and more particularly to methods and resulting structures for making electrical contacts to semiconductor regions within the integrated circuit.
2. Description of the Prior Art
Metallization is a critical processing technique throughout the integrated circuit process. It is particularly critical in the formation of electrical contacts to semiconductor regions in silicon integrated circuits in the one micron and submicron feature size era.
Some years ago the use of direct aluminium electrical contacts to semiconductor regions in silicon semiconductor devices was predominant. However, aluminium with direct contact to silicon leads to metallurgical interactions causing dissolution of silicon by diffusion into the metal and which in turn leads to boundary interface problems. Also, there is movement of aluminium into the silicon region as well. This interaction between aluminum and silicon is a thermally activated process and depends on the temperature and duration of any heating or annealing.
It became common to include a small percentage, such as 1 or 2 per cent of silicon in the deposition of aluminum to overcome or prevent junction shorts caused by the preferential dissolution of silicon into the aluminum. There are contact failures which occur in these aluminum doped with silicon electrical contact systems. These failures are caused by the precipitation of silicon from the aluminum during cooling. These undesirable silicon precipitates also cause increases in the resistance of the electrical contacts.
Further, the precipitate nodules of silicon are formed preferentially at the boundary of the aluminium to silicon interface. These silicon nodules leads to nonohmic contact problems in addition to the obvious increase of resistance problem. These nodules have the characteristic of growing from small seeds to larger nodules during temperature cycling at temperatures greater than about 300.degree. C.
To overcome these problems of direct aluminum to silicon or aluminum-silicon to silicon electrical contacts the use of barriers between these material have widely begun to be used. Self aligned transition and noble metal silicides of metal such as platinum, palladium, cobalt, nickel, titanium, tantalum, tungsten, etc. have been used as the barrier metal. There are problems with this alternative involving metallurgical interaction, consummation of the shallow silicon semiconductor region, etc. Other barrier materials proposed are titanium nitride, titanium carbide, titanium-tungsten, and the like.
Another alternative to the barrier metal is to have the primary contact metal to be a metal, such as tungsten to the semiconductor region. Then the second level of metallurgy is the more conductive aluminium, which is well away from the silicon semiconductor region. This alternative is finding use particularly in the micron and submicron era. However, there is a loss of conductivity in the use of tungsten over aluminium.
There are several processes to deposit metals for electrical contacts to semiconductor regions. The principal deposition technique used for aluminium is by evaporative deposition in a vacuum chamber using an electron beam directed at the source to vaporize the material. Such a technique is described in E. K. Broadbent U.S. Pat. No. 4,495,221. In this Patent aluminum is deposited, with or without a silicon or copper component wherein a two step method is followed. The first step is to deposit at a lower deposition rate and then a second step is done at a higher deposition rate. The temperature is maintained constant during this process. The purpose of the process is to improve step coverage.
Another process that can be used to deposit metal on to semiconductor devices is by sputtering. The A. G. Blachman U.S. Pat. No. 3,856,647 describes a method for sputter coating a thin refractory metal, such as molybdenum onto a substrate which achieves a minimum resistivity and minimum stress. A first thin refractory metal layer is deposited at a first voltage bias on the substrate and then the thicker layer of refractory metal is deposited over the first layer at a second voltage bias that is different than the first voltage. The method is solely involved with reducing stress in the refractory metal layer being deposited.
It is a primary object of this invention to provide a method for depositing an aluminum or aluminum doped with silicon electrical contact to a semiconductor region of an integrated circuit that overcomes the deficiencies of the prior art.
It is a further object of this invention to provide a method for deposition of aluminum or aluminum doped with silicon electrical contact to a silicon semiconductor region in two or more layers which substantially reduces the precipitation and growth of silicon nodules during temperature cycling of greater than about 300.degree. C. at the interface between the silicon semiconductor region and the aluminum electrical contact.